The present invention relates to ohmic heating elements, and more particularly to emission of thermal energy from refractive metal compounds resistive members. The present invention finds particular application for the production of infrared (IR) or thermal images from tightly packed pixel elements formed from these materials.
A major challenge in resistive IR emitter array technology is to produce a high emittance structure that requires relatively little electrical current during operation. The key factors which contribute to high emittance are the density of the pixels which form the array, and the maximum operating temperature of the pixels. High pixel density has been achieved in the prior art using a multi-level pixel structure. The multi-level pixel structure maximizes the radiating area by placing the pixel drive and addressing electronics directly under the a resistive emitting member. High radiance is achieved by fabricating the resistive emitting member of the pixel using a thin, absorbing film, and placing a reflector below this film to direct radiation outward.
The electrical current used by a thermal emitting pixel is strongly linked to the material used to form its resistive emitting member. In prior art systems the designer traded off low current operation for high temperature operation or compromised on other pixel characteristics. For example, metal films used for the resistive member such as platinum, although potentially having good high temperature properties, do not have high resistivities. Thus, platinum resistive members must be patterned into an extremely thin serpentine film to maximize their resistance. Unfortunately, the adhesion of these platinum films is poor, making the pixel structurally weak.
Titanium nitride is another material which has been used to form resistive members in thermal emitters. Titanium nitride has good temperature properties, satisfactory resistance and structural properties, but unfortunately involves sensitive pixel fabrication steps. Specifically, an annealing is typically done during processing of the thermal emitter to stabilize the device for high temperature operation. The resistance of titanium nitride varies considerably in the range of temperatures used for this anneal. This sensitivity can lead to large variations in pixel resistance from array to array and possibly large variations in emmisivity from pixel to pixel in the same array. In fact, the range of resistance of the titanium nitride resistor can in some cases cause pixels to become inefficient or completely inoperative. Titanium nitride resistive members also suffer from some difficulty with lifetime high temperature stability.